Ceramic materials have long been known for their good chemical stability and corrosion resistance. Of the ceramic materials, metallic carbide powders are used to form dies, cutting tools, wear resistant parts and electrical resistors, as well as being used as abrasives in liquids for polishing. For example, cemented tungsten carbides are useful in forming tools and abrasives for machining and grinding of rock, porcelain, glass and metals. Large grain size carbides are acceptable for many grinding applications. However, newer technology requires improved hardness, toughness or both that is produced only by submicrometer metallic carbides or submicrometer solid solution metallic carbides. Traditional production of submicrometer particles is capital intensive and incorporates long grinding or milling times.
Prior methods generally have related to the use of free metals which were carburized in order to produce the metallic carbides. Frequently, metallic oxides, which are naturally occurring, were ground to a very small size, chemically reduced to produce their corresponding free metals, and then carburized to produce the metallic carbides. However, a substantial drawback to using free metals is that it is difficult to make submicrometer free metal particles. Once such particles are made, they are pyrophoric and difficult to handle.
One solution presently offered to this drawback is the use of solid solution carbides, also known as mixed metal carbides, in place of the pure metallic carbides. A solid solution metallic carbide is a carbide that contains an alloy or mixture of two or more metals in a single carbide. Sources used in preparing a solid solution metallic carbide include combinations of individual metallic oxides and alloys of various metals in their oxidative form. For example, tungsten carbide is a much needed ceramic. However, tungsten is a very expensive metal. By replacing part of the tungsten in a tungsten carbide with a cheaper metal such as titanium, a less expensive substitute with similar physical properties can be achieved. Such a product may include a solid solution carbide that has lower cost and weight, yet exhibits improved wear, abrasion and impact resistance. Titanium is a good substitute in tungsten carbide because not only is titanium dioxide (TiO.sub.2) less dense than tungsten trioxide (WO.sub.3), a common starting material for the tungsten carbide, but TiO.sub.2 costs about 1/6 as much as WO.sub.3 at currently published prices.
Physical properties of ceramic articles that incorporate a metallic carbide or a solid solution carbide depend to a great extent upon the grain size of the carbide powder employed. In some applications, carbide powders with very fine grain sizes are quite useful. Such carbides, having mean particle sizes of no more than 1 micrometer and especially from 0.4 to 0.8 micrometer (.mu.m), are known as submicrometer or micrograin carbides. Submicrometer tungsten carbides, for example, are especially useful for various purposes that include fabricating tools or parts for end milling and circuit board drilling applications, and use as reinforcing materials in ceramic metal composites. In addition, the submicrometer particle size carbides have been suggested as being useful in catalytic processes. Furthermore, it is especially preferred that submicrometer carbide particles have a controlled morphology, a narrow size distribution, a well-defined stoichiometry, and are relatively high in purity.
While a variety of processes for preparing these metallic carbide powders are known, many achieve particle sizes well above submicrometer size, especially with regards to the solid solution carbides. The same hardness and resistance to wear for which metallic carbide powders are especially valued also makes their mechanical reduction to smaller sizes by conventional methods, such as grinding or milling, difficult and costly. Accordingly, it is especially desirable that carbide powders be manufactured with a small initial size, rather than attempting to reduce the particle size after the powders have been formed.
One additional drawback related to known technology of making transition metal carbide powders in general, and WC in particular, is related to their use in the manufacture of cemented carbides (e.g., WC-Co). It is well established that grain growth occurs when pure WC is liquid phase sintered with Co to make a cemented carbide. This becomes a problem when an application such as end milling or circuit board drilling requires a very small particle size in the final densified part. One method of dealing with this problem is the use of second carbide phase(s) such as vanadium carbide (VC), titanium carbide (TIC), trichromium dicarbide (Cr.sub.3 C.sub.2) and tantalum carbide (TaC) that act(s) as a grain growth inhibitor during liquid phase sintering. When the particle size of a primary carbide, such as WC becomes very small, however, it becomes extremely difficult to get an intimate and homogeneous distribution of the second (or grain growth inhibiting) carbide phase(s). In view of this problem, it would be very desirable to provide a method by which a novel ultrafine carbide material with an intimate distribution of at least one grain growth inhibiting carbide phase could be produced. It would be even more desirable if the grain growth inhibiting carbide phase(s) could be made in situ during synthesis of the material and if the distribution of said grain growth inhibiting carbide phase(s) was on a size scale less than or equal to the primary carbide phase. It is contemplated that this desirable distribution would include, but not be limited to, atomic scale distribution in the form of a solid solution.
In view of the above-mentioned drawbacks and problems, it would be very desirable to have a method for making submicrometer metallic carbides and submicrometer solid solution metallic carbides more efficiently, and less expensively, than previously known.